Hard disk drives are in widespread use today. They provide an economical means for permanently storing large quantities of data in a relatively small space.
As is well known, such devices typically include one or more rotating magnetic disk surfaces, one or more data transfer heads, and positioning means for positioning the heads over a desired track on the rotating magnetic disks. The positioning means typically includes an arm on which each head is mounted connected to a positioning motor. The coil of the motor is connected to servo-feedback circuitry.
In such systems, there usually is a static bias force asserted against the data transfer head and its associated pickup arm when they are at rest over a particular track on a rotating disk. This force usually tends to move the head off of the particular track.
One source of this force is wind which is created by the rotating disk. The force is usually directed radially outwardly from the rotating disk. Its strength increases as the head gets closer to the perimeter of the disk.
Another source of the force is stresses created by the cable which is typically used to connect the moving data transfer head to a stationary portion of the hard disk drive. Both the magnitude and direction of this stress generally vary depending on the position of the head.
A still further source of the static bias force is torsional stress caused by the viscosity of lubricants used in the pivotal joint which supports the moving arms on which the heads are mounted. The direction of this stress is usually opposite of the direction in which the head traveled. The magnitude varies somewhat dependent upon position and also the direction from which the head approached the track.
Existing control circuitry, to applicant's knowledge, makes no attempt to anticipate and compensate for this combined bias force before the data head is positioned over the desired track. As a result, the head is usually pulled off of the desired track after initial positioning, requiring compensation to be applied by the control circuitry to bring the head back. This delays the time when data transfer to or from the desired track may safely take place.
Another problem with existing positioning circuitry lies in the sensing means which are used to detect actual head velocity. More specifically, such sensing circuitry generally does not work well both at high and low velocity. As a result, low velocity measurements are often inaccurate, injecting instability into the system. This further delays the point in time when the system is ready to safely transfer data.
In short, there has been a continuing need for control circuitry which rapidly and accurately positions a hard disk drive head over a desired track and, more specifically, which does not allow added delays to be caused by the static bias force exerted on the head and/or imperfections in the velocity sensing circuitry.